Permanent magnet and method of manufacture thereof



Nov. 27, 1951 c. GUILLAUD 2,576,679

PERMANENT MAGNET AND METHOD OF MANUFACTURE THEREOF Filed Nov. 9, 1946 INVE N TOR Char/es Gui/laud HIS ATTORNE Y5 Patented Nov. 2

PERMANENT MAGNET AND METHOD OF MANUFACTURE THEREOF Application November9, 1946, Serial No. 708,926 In France August 2, 1939 Section 1, PublicLaw 690, August 8, 1946 Patent expires August 2, 1959 4' Claims. (01.175-21) A number of metallic alloys or of defined compounds are knownwhich possess special magnetic qualities called ferro-magneticproperties. Amongst these alloys or compounds only a small number possesthe characteristics which are wanted for the manufacture ofpermanentmagnets, namely a coercive force and residual magnetisationintensity, which are large enough for the purposes contemplated.

Some alloys of iron and other metals, notably cobalt, nickel, aluminium,possess such special properties when they are in the shape of ingots,rods, forged or cast pieces, or when they are given such properties by aconvenient heating treatment. For these reasons they are commonly usedunder these various shapes as permanent magnets.

It has also been proposed to manufacture ma nets by starting frompowders of such alloys which show, notably when they are in the state ofrough castings, high coercive forces. Such a process permits using thewaste or scraps of these substances, which otherwise could only bereused through a melting process, but the so obtained magnets have acoercive force the value of which is only at the most and as a rule,lower than the one which is obtained when a magnet of the same shape ismanufactured by casting or by forging an ingot ora rod.

On another hand it has already generally been disclosed that manganeseand bismuth alloys possess term-magnetic properties, and especially thatthey possess a Curie-point, but no data have been given as to theirspecific magnetisation, their coercive force or their residualmagnetisation intensity and such alloys have never been used for makingmagnets.

The applicant has discovered the most surprising act that amongst allthe alloys of manganese and bismuth, the well defined alloycorresponding to the chemical formula: Mn Bi, the existence of which hasbeen disclosed, but which does not seem at present as having beenisolated, possesses very particular ferro-magnetic properties. Theapplicant having succeeded in isolating the alloy Mn Bi, has discoverednot only that it is ferromagnetic but also that the said alloy which inmassic state, i. e., in the state of ingot or cast piece, possesses as arule but exceedingly small coercive forces and residualmagnetisationintensities, can be given, especially when the crystals ofthe same are separated from each other, e. g. by comminuting itintopowder, extraordinarily high coercive force and residual magnetisationintensity.

The above discovery has led the applicant according to the presentinvention, to use the alloy Mn Bi which he had been able to isolate andto study, for making permanent-magnets and he has invented a processwhich allows of taking advantage of the extraordinary properties of thesaid alloy for the manufacture of permanentmagnets which possessparticularly valuable characteristics.

According to the invention an alloy of manganese and bismuth ismanufactured which has a chemical composition which is as close aspossible to the chemical formula Mn Bi. This alloy is preferablycomminuted down to the state of a fine powder and the grains thereof aresubmitted to a selection in order to only keep the grains the chemicalcomposition of which substantially corresponds to the formula Mn Bi; theso selected grains are then brought to the desired solid shape by anyknown agglomerating means, preferably by submitting them to anorientation field, and they are finally subjected to a very strongorientating magnetizing field which advantageously has the samedirection as the orientating field.

The manufacture of the initial binary alloy of manganese and bismuth canbe carried on, e. g. by melting together the two bodies of theseelements. Such a, melting can be carried on either at a hightemperature, comprised for instance between 700 and 1,250 C. or on thecontrary, at a relatively low temperature (from 260 C. and over) bydissolving solid manganese in molten bismuth. The melting process can e.g. be carried on by heating in a high frequence electric furnace, apowdered mixture of such weights of manganese and bismuth that they arerespectively proportioned to the molecular weights of manganese andbismuth, the sizes of the grains of the elements being so chosen inrelation with the frequence of the furnace, according to a practiceknown per se, that the fusion takes place. Whatever may be the manner inwhich the manganese-bismuth alloy is manufactured, it appears aspreferable to aim at the formation of an alloy the composition of whichis as close as possible to the chemical formula of the compound Mn Bi.

It may be advantageous to submit the ingots of Mn Bi alloy to anannealing treatment at a temperature comprised between 150 and 600 C.during a period of time of 2 to hours, in order to obtain big crystals.The magnetic properties of the final magnets will be increased thereby.The alloy is then crushed down to very fine grains, for instance of theorder of M of millimeter. As it has been disclosed hereabove thesegrains are eventually submitted to a selection in order to retain onlythe ones which correspond or substantially correspond to the chemicalformula of the compound Mn Bi. This selection can be made by means ofknown processes based on the differences in the densities of the grains,or better by a magnetic action exerted on the latter. Such a selectionis especially convenient when the chemical composition of the initialalloy substantially diverges from the one of the compound Mn Bi.

The grains which have been separated by this selection are thenagglomerated by any known means used in the art. For instance they canbe caused to fioat in the body of a more or less liquid binder such asthe ones which are commonly used for the manufacture of agglomeratedmagnets when starting from known alloy powders for making magnets, orthey are mixed therewith. Preferably, during the time the grains arestill movable, the ensemble of the liquid binder and grains is submittedto a magnetic field capable of setting the grains in a predetermineddirection, after which the binder is allowed to solidify. The magneticfield which is used for giving a predetermined direction to the grains(for orientating the grains) can be of the same order of intensity asthe ones used for the same purpose for known other substances utilizedin powdered state for making permanentmagnets. Whatever may be theprocess which is used'for the agglomeration, the latter shouldpreferably be carried on at relatively low temperatures-lower than theCurie-point of the compound Mn Biin order that it is possible to give tothe grains a predetermined direction by means of a magnetic field. Thesaid agglomeration can eventually be made in a magneticmetal casingpossessing the external shape which is required for the finished magnet,and in which the magnet can remain enclosed in view of being protectedagainst external agents capable of modifying its composition.

In the accompanying drawing, the only figure is a perspective view of amagnet made of agglomerated powder according to the invention, enclosedin a casing.

In such drawing, I represents the casing in which the magnet has beenagglomerated and 2 represents the pole faces of the magnet.

The magnetisation of the agglomerate is then performed by subjectingthis agglomerate to a magnetizing field preferably having the samedirection as the one which has been used for giving a predetermineddirection to the .grains (orientating the grains) but preferablypossessing a much higher intensity as the one commonly used for themagnetisation of permanentmagnets. It has been acknowledged that whenthe agglomerate has been subjected under the hereabove disclosedconditions, to a magnetizing field of the order of 4,000 oerstedwhichfield is already stronger than the ones commonly used-it possesses thefollowing magnetic properties (which correspond to a filling coefficientof the agglomerate equal to one unit):

Magnetisation intensity=about 6,300 gauss Residual magnetisationintensity after the field has been suppressed=about 1,300 gaussCorrespondent coercive force=500 oersted.

Such values correspond to very low characteristics of permanent magnets.As a set ofi if under the same conditions, the agglomerate is subjectedto a much stronger field, for example of the order of 20,000 oersted,the remarkable following results are obtained:

Magnetisation intensity=about 6,800 gauss Residual magnetisationintensity after the field has been suppressed=about 5,700 gaussCorrespondent coercive force=4,000 oersted.

The magnetisation intensity is thus of the same order of magnitude as inthe preceding case, but the residual magnetisation intensity is morethan four times the previously reached figure, and the coercive force iseight times higher.

Such values of residual magnetisation and of coercive forces correspondto very remarkable characteristics for permanent magnets and they areconsiderably higher than the ones of known magnets. It is indeed knownthat the value of a magnet (setting aside the shape coeflicient) ischaracterised by these two values taken together. Now as a matter ofindication, the substances having a minimal hysteresis surface, whichare used for making magnets, possess a residual magnetisation intensityof 12,000 gauss and a "coercive force of 60 oersted; the substanceshaving a maximal hysteresis surface possess a residual magnetisationintensity of 5,000 gauss and a coercive force of about 800 oersted. Forinstance the well known nickel-aluminium alloys used for makingpermanent alloys, possess a residual magnetisation intensity of theorder of 6,000 gauss, i. e. close to the one stated hereabove formagnets made of the compound Mn Bi, but the value of their coerciveforce is, on the con trary, from six to ten times lower than that of4,000 oersted obtained with the said Mn Bi alloy.

The figures quoted hereabove for the magnetic characteristics of thealloy Mn Bi do correspond to figures which have been measured onagglomerates, but as it has been said, they have reference to a fillingcoefiicient of the agglomerate which is equal to one unit, in order toset alight the characteristics of the substance itself. The figures thusobtained on an agglomerate are obviously a function of the fillingcoefficient of the same. It can roughly be said that the residualmagnetisation intensity of the agglomerate nearly proportionally varieswith that coemcient; on the contrary. the coercive force varies butlittle and remains rather close to the value corresponding to a fillingcoeflicient equal to 1.

On the other hand it is evident that the magnitude of 20,000 oerstedwhich has been quoted as being the value of the magnetizing field withwhich the remarkable results stated hereabove have been obtained, is inno manner limitative and is only given by way of example. It can bedeparted from said value but one should always remain in a range ofmagnitudes, which are notably higher than those of the present commonpractice. As a mere indication a rule can be given that the magnetizingfield which has to be used for manufacturing magnets according to theinvention should preferably have a magnitude which is at least four tofive times the one which corresponds to the saturation of the magnet.When needed, preliminary tests will allow to fix it in each particularcase.

On the other hand, the above given figures have been taken fromagglomerates the alloy grains of which had been given predetermineddirection by means of a magnetic field during the step of agglomeration.Such an orientating practice can be dispensed with but in such a case,the results show lower figures. The magnitudes obtained under the samemagnetizing conditions as hereabove disclosed but without orientatingthe grains, only reach 30% as concerns the residual magnetisationintensity and 40% as concerns the coercive force, of the values whichare obtained when the grains are submitted to an orientating operation.

I claim:

l. A permanent magnet possessing a high residual magnetization and acoercive force of at least 4,000 oersteds, said permanent magnet beingmade from a massive binary alloy of manganese and bismuth in which theproportions of manganese and bismuth substantially correspond to themolecular weights of said elements, which massive alloy has been crushedto a very fine powder from which the alloy crystals, the chemicalcomposition of which is substantially Mn Bi, have been selected andagglomerated to the desired shape, the agglomerate having been thensubmitted to a strong magnetizing field.

2. A permanent magnet possessing a high residual magnetization and acoercive force of at least 4,000 oersteds, said permanent magnet beingmade from a massive binary alloy of manganese and bismuth in which theproportions of manganese and bismuth substantially correspond to themolecular weights of said elements, which massive alloy has been crushedto a very fine powder from which the alloy crystals, the chemicalcomposition of which is substantially Mn Bi, have been selected andagglomerated to the desired shape, the agglomerate having been thensubmitted to a strong magnetizing field.

3. The process of making a permanent magnet, which comprises forming amassive binary alloy of manganese and bismuth in which the proportionsof manganese and bismuth substantially correspond to the molecularweights of said elements, annealing said massive manganese bismuth alloyto increase the crystal size thereof, crushing said alloy in massivestate to a very fine powder, selecting from said powder the alloycrystals, the chemical composition of which is substantially Mn Bi,mixing said crystals with a liquefied, setting binder, submitting themixture to a magnetic field before the liquefied binder has setcompletely, thereby orienting the crystals,

causing the binder to set to form an aggiomerate. and submitting theagglomerate to a strong magnetizing field.

4. The process of making a permanent magnet, which comprises forming amassive binary alloy of manganese and bismuth in which the proportionsof manganese and bismuth substantially correspond to the molecularweights of said elements, annealing said massive manganese bismuth alloyto increase the crystal size thereof, crushing said alloy in massivestate to a very fine powder, selecting from said powder the alloycrystals, the chemical composition of which is substantially Mn Bi,mixing said crystals with a liquefied, setting binder, submitting themixture to a magnetic field before the liquefied binder has setcompletely, thereby orienting the crystals, causing the binder to set toform an agglomerate, and submitting the agglomerate to a magnetizingfield of about 20,000 oersteds, the direction of which is the same asthat applied for orienting the crystals.

CIlARLES GU'ILLAUD.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,739,752 Elmen Dec. 17, 19291,772,771 Fetkenheuer Aug. 12, 1930 1,997,193 Kato et a1 Apr. 9, 19352,031,947 Haglund Feb. 25, 1936 2,076,230 Gillis Apr. 6, 1937 2,110,967Andrews Mar. 15, 1938 2,188,091 Baermann Jan. 23, 1940 2,247,804 FausJuly 1, 1941 2,381,023 Wulif Aug. 7, 1945 FOREIGN PATENTS Number CountryDate 432,152 Great Britain July 22, 1935 OTHER REFERENCES ChemicalAbstracts, Vol. 34 (1940). page 4315.

"Practical Metallurgy by Sachs and Van Horn, published by the Americansociety for Metals. 1940, pages 123 and 124.

Proceedings of the Royal Society, Vol. 117, page 681 (1927).

Certificate of Correction Patent No. 2,576,679 November 27, 1951 CHARLESGUILLAUD It; is hereby certified that error appears in the printedspecification of the above numbered patent requiring correction asfollows:

Column 5, line 31, after been insert annealed to increase the crystalsize thereof and than;

and that the said Letters Patent should be read as corrected above, sothat the same may conform to the record of the case in the PatentOlfiee.

Signed and sealed this 25th day of March, A. D. 1952.

THOMAS F. MURPHY,

Assistant C'ommz'ssioner of Patents.

